Paranasal sinuses, which communicate with the nasal passages through the sinus ostia, are essentially sterile structures, sterility being maintained by a healthy epithelium with normal actively beating cilia. Irritants, including viruses and bacteria, are trapped in mucus and cilia to allow the clearance of sinuses through the natural ostia into the nasal cavity. Interference with this normal physiological function results in inflammation and infection within the sinus cavities. All of the sinuses are subjected to the same environmental as well as physiological stimuli; thus it is uncommon for a single sinus to be infected and for the others to remain entirely normal. Allergic and non-allergic vasomotor rhinitis should be differentiated from chronic bacterial rhinosinusitis. The understanding of these diseases cannot be separated from the physiological function of the sinus mucosa.